In recent years, the development of steel stronger than ever is wanted to fulfill the requirements of scale-up of steel structures such as marine structures and of reduction in line pipe laying cost. Since the steels having about 570 MPa (N/mm2) or larger tensile strength induce martensitic or bainitic transformation resulting from quenching, thus giving poor toughness of as-quenched steels, they are often improved mainly in the toughness before practical applications by applying succeeding tempering treatment to precipitate carbide from super-saturation solid solution carbon.
That type of quenched and tempered steel plates is conventionally manufactured by directly quenching after rolling, followed by tempering, as disclosed in, for example, JP-B-55-49131, (the term “JP-B” referred to herein signifies the “Examined Japanese Patent Publication”).
The process of tempering in the disclosed technology, however, takes a long time for heating the steel plate and holding the temperature thereof so that the tempering has to be given in a separate line from the quenching manufacturing line. As a result, the transfer of the steel plate to the separate line takes unnecessary time in view of metallurgy. Therefore, the disclosed technology needs an improvement from the point of productivity and manufacturing cost.
To solve the above problems, Japanese Patent No. 3015923, Japanese Patent No. 3015924, and the like disclose methods for manufacturing high strength steel that allows tempering thereof in the same manufacturing line of quenching owing to the achieved rapid and short time of tempering, that significantly increases the productivity of quenched and tempered steel plate, thus improving the productivity and the manufacturing cost, and that provides a steel plate tougher than conventional quenched and tempered steel plate also in view of material.
The material which is rapidly tempered in a short time, disclosed in the above Japanese Patent No. 3015923 and Japanese Patent No. 3015924, however, have a drawback of being unable to respond to a severe toughness requirement in a cold district. Accordingly, a method for manufacturing further tough high strength steel was desired.
Furthermore, high tensile strength steels used as tanks, penstocks, and the like often achieve the prevention of occurrence of deformation and brittle fracture of structures by applying PWHT after the welding which is given on fabricating the structures, thereby conducting relief of the residual stress, softening of the weld-hardened part, and desorption of hydrogen in the weld-hardened part.
Increase in the size of steel structures such as tanks and penstocks is a trend in recent years, thus the need of increased strength and thickness of steels increases. Increase in the strength and the thickness of steels, however, also raises severe PWHT conditions of higher temperature and longer time, thereby often inducing decrease in strength and toughness after the treatment.
To cope with these problems, JP-A-59-232234, (the term “JP-A” referred to herein signifies the “Unexamined Japanese Patent Publication”), JP-A-62-93312, JP-B-9-256037, JP-B-9-256038, and the like disclose methods for manufacturing steel plate having excellent strength and toughness after PWHT, by optimizing alloying elements, applying work-heating treatment technology, or utilizing heat treatment before PWHT.
The methods disclosed in JP-A-59-232234, JP-A-62-93312, JP-B-9-256037, JP-B-9-256038, and the like have, however, a problem that the steel cannot respond to the severe request of strength and toughness characteristics after PWHT, which request is given for the case of cold-district services, and the like. Therefore, there has been a desire for a method of manufacturing high tensile strength steel plate that has superior balance of strength and toughness after PWHT to that of conventional steel plates.